Upconversion luminescence and temperature sensing properties of Ho3+,Yb3+-codoped Bi2WO6

Abstract

Ho³⁺ and Yb³⁺-codoped Bi₂WO₆ upconversion luminescent materials at different concentrations were prepared via a high-temperature solid-phase method. The X-ray diffraction patterns showed that Ho³⁺ and Yb³⁺ doping basically did not affect the orthorhombic crystal system structure of the Bi₂WO₆ matrix material. Scanning electron microscopy images showed that 3%Ho³⁺,10%Yb³⁺:Bi₂WO₆ consisted of irregular bulk particles with sizes in the range of 0.5–2 μm and some powder agglomeration. SEM mapping and EDS measurements of the powder showed that the elements were relatively uniformly distributed. Under 980 nm excitation, the emission intensity of Ho³⁺ was the largest for the 3%Ho³⁺- and 10%Yb³⁺-doped sample. With an excitation power ranging from 45 mW to 283 mW for the 3%Ho³⁺,10%Yb³⁺:Bi₂WO₆ sample, the relationship between the luminescence intensity and pump power was determined; the results indicated that the Ho³⁺ (538 nm, 546 nm, 660 nm, 756 nm) emission peaks originated from two-photon absorption. In the temperature range of 298 K–573 K, under 980 nm laser excitation, the maximum absolute temperature sensitivity S_a was 0.029% K⁻¹ (373 K), the maximum relative temperature sensitivity S_r was 0.034% K⁻¹ (348 K) for the Ho³⁺ thermally coupled energy levels ⁵F₄/⁵S₂, and the minimum temperature resolution δT was 1.2857 K (298 K). Under the same conditions, the maximum S_a was 51.02% K⁻¹ (573 K), the maximum S_r was 1.85% K⁻¹ (523 K) for the Ho³⁺ nonthermally coupled energy levels ⁵F₅/⁵F₄, and the minimum δT is 0.2477 K (448 K). The colour coordinates showed that the luminescence of the 3%Ho³⁺,10%Yb³⁺:Bi₂WO₆ sample gradually shifted from the green region to the red region with increasing temperature.